1. Field of the Invention
This invention relates to a hydraulic circuit for construction machines, and more particularly to a cutoff function-carrying hydraulic circuit for construction machines, adapted to improve the suspension capacity thereof by disengaging the cutoff function and increasing a maximum discharge pressure of the hydraulic circuit when the power and speed of a working unit are required at once in load suspending work, and stump digging work or boulder raising work.
2. Description of the Related Art
A hydraulic excavator of the related art is used to suspend a load in some cases by a boom extending operation. First related art hydraulic circuit for construction machines shown in FIG. 7 will now be described.
A discharge pipe line 1a of a variable displacement pump 1 is connected to a boom cylinder 3 via a direction change-over valve 2 for a boom, and a relief valve 4 for setting a maximum discharge pressure (set pressure P1) to the discharge pipe line 1a of the variable displacement pump 1. A discharge oil of a control pump 5 is set to a predetermined pressure by a fixed relief valve 6, and sent to a volume control unit 9 for the variable displacement pump 1 via a torque variable control valve 7 from a cutoff valve 8. The torque variable control valve 7 receives a discharge pressure P of the variable displacement pump 1, controls a discharge oil of the control pump 5, which has been set to a predetermined level, to such a control pressure that permits volume V (cc/rev) of the variable displacement pump 1 to set constant the torque K2 shown by P.multidot.V, and outputs the resultant control pressure. Namely, the control pressure becomes rectangularly hyperbolic as shown by the torque K2 in FIG. 8. The cutoff valve 8 receives a control pressure outputted from the torque variable control valve 7, and outputs when the discharge pressure P of the variable displacement pump 1 reaches a level in the vicinity of a set level P1 (i.e. Pa) of the relief valve 4 such a control pressure that makes the volume V of the variable displacement pump 1 decrease gradually and attain a minimum level V1 as shown by a curve C in FIG. 8. The volume control unit 9 comprises a volume control valve and a volume control cylinder (neither of which is shown) which are adapted to receive a control pressure outputted from the cutoff valve 8, and control the volume V of the variable displacement pump 1 as shown by K2 and C in FIG. 8.
The operation of the first related art hydraulic circuit will now be described with reference to FIGS. 7 and 8.
The volume V of the variable displacement pump 1 is controlled as shown by the torque curves K2 and C in FIG. 8, in accordance with the discharge pressure P of the variable displacement pump 1 which is determined depending upon a load pressure working on the boom cylinder 3. In FIG. 8, the volume V of the pump is taken in the direction of the lateral axis. When a discharge rate Q (m.sup.3 /min) of the pump is taken on the same axis, the torque curves shown by K2 and C turn into horsepower curves. Therefore, loss torque K1 (i.e. P1.multidot.V1) which the variable displacement pump 1 relieves at a point A at the cutoff time decreases as compared with that K2 (i.e. P1.multidot.V2) which the variable displacement pump 1 relieves at a point B at the cutoff operation stopping time. Consequently, the loss horsepower which the variable displacement pump 1 relieves at the point A decreases as compared with that which the variable displacement pump 1 relieves at the point B, so that the saving of energy is attained.
Second related techniques (disclosed, for example, in Japanese Patent Publication No. 72437/1994) shown in FIG. 9 will now be described. In the parentheses shown after the names of constituent elements of this related art hydraulic circuit, the names of corresponding constituent elements of the present invention will be inserted once each, and the descriptions will thereafter be given by referring to the name of constituent elements of the present invention with the constituent elements of this related art hydraulic circuit equivalent to those of the first related art hydraulic circuit designated by the same reference numerals to omit the descriptions thereof.
In a hydraulic circuit for a working unit of a hydraulic excavator, a return oil passage 10 passing through each direction change-over valve is connected to a tank 12 via a restriction 11. A discharge oil of a hydraulic pump (control pump) 5 the pressure in which is set at a predetermined level is sent to a pilot pressure receiving member 14b of a variable relief valve (relief valve) 14 via a solenoid valve 13, and further from one side of a shuttle valve 15 to a volume control unit 16 of a variable displacement hydraulic pump (variable displacement pump) 1. The other side of the shuttle valve 15 is connected to an upstream side of the restriction 11 provided in the return oil passage 10. The solenoid valve 13 is excited when a switch 19 provided on a free end of an operating lever 18 is pressed, and it is thereby shifted to a position b to cause the control pump 5 to be connected to the pilot pressure receiving member 14b and the first-mentioned side of the shuttle valve 15. When the hand pressing the switch 19 is removed therefrom, the solenoid valve is deexcited to be shifted to a position a, and connect the pilot pressure receiving member 14b and the first-mentioned side of the shuttle valve 15 to the tank 12. The relief valve 14 is set to a normal set level P1 when a pilot pressure is not supplied to the pilot pressure receiving member 14b, and shifted to a higher set level P2, which is higher than the normal set level P1, when a pilot pressure is supplied thereto.
The operation of the second related art hydraulic circuit will now be described. When regular excavation work is carried out by the hydraulic excavator, the switch 19 is not pressed, so that the solenoid valve 13 is deexcited, and takes the position a. Accordingly, the pilot pressure receiving member 14b and the first-mentioned side of the shuttle valve 15 are drained. Consequently, the relief valve 14 comes to have a normal set pressure P1, and a pressure oil in the portion of the return oil passage 10 which is on the upstream side of the restriction 11 works from the second-mentioned side of the shuttle valve 15 on the volume control unit 16. Owing to this operation, the variable displacement valve 1 is controlled so that, when none direction change-over valves, such as a direction change-over valve 2 for a boom is operated, a flow rate in the return oil passage 10 increases to cause the volume V of the variable displacement pump 1 to become minimal, and so that, when any one of the direction change-over valves, such as the direction change-over valve 2 for a boom is operated, a flow rate in the return oil passage 10 becomes zero to cause the volume V of the variable displacement pump 1 to become maximal.
In order to use the hydraulic excavator as a crane for load suspending work, the switch 19 of the operating lever 18 is pressed, so that the solenoid valve 13 is excited to take the position b. Accordingly, a pilot oil from the control pump 5 flows to the pilot pressure receiving member 14b to increase the oil pressure to a higher set level P2 as shown in FIG. 10, so that lifting power increases. Moreover, since a control pressure of the control pump 5 is applied from the first-mentioned side of the shuttle valve 15 to the volume control unit 16, the volume V of the variable displacement pump 1 is set to a lower level V3 as shown in FIG. 10.
However, these related techniques have the following problems.
(1) When load suspending work is carried out according to the first related techniques, the discharge pressure P of the pump increases, i.e., the load suspending work is necessarily carried out at a discharge pressure in the vicinity of a point A in FIG. 8. Consequently, the volume V of the pump decreases to cause a work speed to decrease, and an operation efficiency therefore also decreases. The discharge pressure P of the pump is set to P1 by the relief valve 4, and any higher suspending capability cannot be obtained. Therefore, the suspending capability becomes insufficient, and satisfactory load suspending work cannot be carried out. PA1 (2) According to the second related techniques, when the switch 19 is pressed inadvertently or when a part of an operator touches the same while regular excavation work is carried out with a boom lowered, the pressure in the relief valve 14 increases up to the higher set level P2 against the operator's will to cause the excavation power to increase, and the volume of the variable displacement pump 1 decreases to a lower level V3 to prevent the obtainment of a sufficient working speed. Therefore, the operation efficiency of the operator decrease. Moreover, since hydraulic machines on the boom lowering side require pressure resistance in the same manner as those on the boom lifting side, so that the cost increases. PA1 (3) When the direction change-over valve 2 for a boom is not operated for carrying out regular excavation work in the second related techniques, the volume V of the variable displacement pump 1 reaches a minimum level Vmin as shown in FIG. 10. When the direction change-over valve 2 for a boom is operated, the volume V of the variable displacement pump 1 reaches a maximum level Vmax. When the switch 19 is pressed during suspension work, not only the volume V of the variable displacement pump 1 changes from the minimum level Vmin or maximum level Vmax to a lower level V3 but also the pressure in the relief valve changes suddenly from the normal set level P1 to the higher set level P2. Since the volume V of the variable displacement pump 1 and the set pressure of the relief valve thus change suddenly, the degree of a shock given to each hydraulic machine and further to the construction machine as a whole increases to cause the durability of the equipment to lower.